Nitrogen production method and apparatus

Abstract

Method and apparatus for distilling nitrogen from a gaseous mixture containing nitrogen and oxygen. Oxygen-enriched bottoms liquid is partially vaporized within a first heat exchanger to condense part of the column overhead to produce reflux. Thereafter, the partially vaporized oxygen-enriched liquid is phase separated. A second oxygen-enriched liquid stream composed of at least part of the liquid phase is used to substantially condense all or part of the vapor stream derived from said phase separation, thereby to form a nitrogen-rich liquid stream. At least part of the nitrogen-rich liquid stream is reintroduced into the column to increase nitrogen recovery. The second oxygen-enriched liquid stream is then used to condense a second part of the reflux for the column.

1 . A method of separating a gaseous mixture comprising nitrogen and oxygen to produce a nitrogen product, said method comprising:
 introducing a purified, pressurized and cooled gaseous stream into a rectification column to produce an overhead nitrogen-rich vapor and an oxygen-enriched liquid bottoms;   depressurizing a first oxygen-enriched liquid stream composed at least in part of the oxygen-enriched liquid bottoms, partially vaporizing the first oxygen-enriched liquid stream within a first heat exchanger, disengaging a vapor phase from a liquid phase formed by the partial vaporization of the first oxygen-enriched liquid stream, depressurizing a second oxygen-enriched liquid stream composed at least in part of the liquid phase and partially vaporizing the second oxygen-enriched liquid stream through indirect heat exchange with at least a portion of a vapor phase stream composed of the vapor phase within a second heat exchanger, thereby substantially condensing at least a portion of the vapor phase stream to form a nitrogen-rich liquid stream;   condensing a first part of a column overhead nitrogen-rich stream composed of the overhead nitrogen-rich vapor in the first heat exchanger, condensing a second part of the overhead nitrogen-rich stream in a third heat exchanger through indirect heat exchange with the second oxygen-enriched liquid stream after having been partially vaporized, thereby further vaporizing the second oxygen-enriched liquid stream, and returning at least part of the condensed column overhead nitrogen-rich stream to the rectification as reflux;   introducing at least part of the nitrogen-rich liquid stream into the rectification column, above the purified, pressurized and cooled gaseous stream; and   producing a product nitrogen stream from part of the overhead nitrogen-rich vapor.   
     
     
         2 . The method of  claim 1 , further comprising:
 subcooling the first oxygen-enriched liquid stream through indirect heat exchange with the nitrogen product stream and a waste stream composed of a vaporized fraction of the second oxygen-enriched liquid stream after having been further vaporized; and   cooling a compressed and purified stream, composed of the gaseous mixture, by indirect heat exchange with the nitrogen product stream and the waste stream after having subcooled the oxygen-enriched stream, thereby to form at least a portion of the purified, pressurized and cooled stream from the compressed and purified stream.   
     
     
         3 . The method of  claim 2 , wherein:
 the waste stream and the nitrogen product stream indirectly exchange heat with the compressed and purified stream within a main heat exchanger;   the waste stream is partially warmed within the main heat exchanger and is then expanded with the performance of work to generate an exhaust stream; and   the exhaust stream is reintroduced into the main heat exchanger and fully warmed to refrigerate the cryogenic rectification process.   
     
     
         4 . The method of  claim 1 , wherein pressure of the nitrogen-rich liquid stream is adjusted prior to its being introduced into the rectification column. 
     
     
         5 . The method of  claim 4 , wherein the pressure of the nitrogen-rich liquid stream is adjusted by mechanically pumping the nitrogen-rich liquid stream. 
     
     
         6 . The method of  claim 4 , wherein the pressure of the nitrogen-rich liquid stream is adjusted by valve expanding the nitrogen-rich liquid stream. 
     
     
         7 . The method of  claim 1 , wherein:
 a first part of the vapor phase stream is substantially condensed within the second heat exchanger to form the nitrogen-rich liquid stream; and   a second part of the vapor phase stream is warmed, compressed and cooled and recycled back to the rectification column.   
     
     
         8 . The method of  claim 7 , wherein:
 the first oxygen-enriched liquid stream is subcooled through indirect heat exchange with the nitrogen product stream and the waste stream; and   the compressed and purified stream is combined with the second part of the vapor phase stream after having been compressed to form a combined, compressed and purified stream and the combined compressed and purified stream is cooled by indirect heat exchange with the nitrogen product stream and the waste stream after having subcooled the first oxygen-enriched liquid stream and the second part of the nitrogen-rich vapor stream prior to its compression, thereby to form the purified, pressurized and cooled stream from the combined compressed and purified stream;   whereby the second part of the vapor phase stream is cooled and recycled back to the rectification column by being combined with the compressed and purified stream.   
     
     
         9 . The method of  claim 8 , wherein:
 the waste stream, the nitrogen product stream and second part of the vapor phase stream indirectly exchange heat with the compressed and purified stream within a main heat exchanger;   the waste stream is partially warmed within the main heat exchanger and is then expanded with the performance of work to generate an exhaust stream; and   the exhaust stream is reintroduced into the main heat exchanger and fully warmed to refrigerate the cryogenic rectification process.   
     
     
         10 . An apparatus for separating a gaseous mixture comprising nitrogen and oxygen to produce a nitrogen product, said apparatus comprising:
 a rectification column connected to the main heat exchanger for rectifying a purified, pressurized and cooled stream composed of the gaseous mixture to produce an overhead nitrogen-rich vapor and an oxygen-enriched liquid bottoms;   a first valve for depressurizing a first oxygen-enriched liquid stream composed at least in part of the oxygen-enriched liquid bottoms;   a first heat exchanger connected to the first valve for partially vaporizing the first oxygen-enriched liquid stream;   a phase separator connected to the first heat exchanger for disengaging a vapor phase from a liquid phase formed by the partial vaporization of the first oxygen-enriched liquid stream;   a second valve connected to the phase separator for depressurizing a second oxygen-enriched liquid stream composed at least in part of the liquid phase;   a second heat exchanger connected to the second valve and to the phase separator for partially vaporizing the second oxygen-enriched liquid stream through indirect heat exchange with at least a portion of a vapor phase stream composed of the vapor phase, thereby to substantially condense at least a portion of the vapor phase stream and to form a nitrogen-rich liquid stream;   a third heat exchanger connected to the second heat exchanger for further vaporizing the second oxygen-enriched liquid stream;   the rectification column connected to the first heat exchanger and the third heat exchanger for condensing at least a portion of a column overhead nitrogen-rich stream composed of the overhead nitrogen-rich vapor and returning at least part of the column overhead nitrogen-rich stream after having been condensed to the rectification column as reflux;   the second heat exchanger connected to the rectification column for introducing at least part of the nitrogen-rich liquid stream into the rectification column, above the purified, pressurized and cooled stream; and   means for extracting a product nitrogen stream composed of part of the overhead nitrogen-rich vapor.   
     
     
         11 . The apparatus of  claim 10 , further comprising:
 a main heat exchanger to cool a compressed and purified stream composed of the gaseous mixture and thereby to form at least a portion of the purified, pressurized and cooled stream;   a subcooler connected to the rectification column so that the first oxygen-enriched liquid stream is subcooled through indirect heat exchange with the nitrogen product stream and a waste stream composed of a vapor fraction of the second oxygen-enriched liquid stream after having been further vaporized; and   the main heat exchanger also connected to the subcooler and configured so that the compressed and purified stream is cooled by indirect heat exchange with the nitrogen product stream and the waste stream after having subcooled the first oxygen-enriched stream.   
     
     
         12 . The apparatus of  claim 11 , wherein:
 the main heat exchanger is configured such that the waste stream and the nitrogen product stream indirectly exchange heat with the compressed and purified stream, the waste stream partially warms within the main heat exchanger and an exhaust stream fully warms within the main heat exchanger to refrigerate the apparatus; and   an expander is connected to the main heat exchanger so that the waste stream after having partially warmed is expanded within the expander with the performance of work to generate the exhaust stream.   
     
     
         13 . The apparatus of  claim 10 , wherein a pump is interposed between the second heat exchanger and the rectification column to pressurize the nitrogen-rich liquid stream after having been substantially condensed prior to its introduction into the rectification column. 
     
     
         14 . The apparatus of  claim 10 , wherein a third valve is interposed between the second heat exchanger and the rectification column to reduce the pressure of the nitrogen-rich liquid stream prior to its introduction into the rectification column. 
     
     
         15 . The apparatus of  claim 11 , wherein:
 the second heat exchanger is connected to the phase separator so that a first part of the vapor phase stream is substantially condensed within the second heat exchanger to form the nitrogen-rich liquid stream;   a compressor is connected in flow communication with the phase separator and to the main heat exchanger and the main heat exchanger also configured so that a second part of the vapor phase stream is warmed within the main heat exchanger and compressed within the compressor; and   the main heat exchanger simultaneously in flow communication with the compressed and purified stream and the compressor such that the second part of the vapor phase stream combines with the compressed and purified stream to form a combined compressed and purified stream and the combined compressed and purified stream is cooled within the main heat exchanger to form the purified, pressurized and cooled stream.   
     
     
         16 . The apparatus of  claim 15 , wherein the main heat exchanger is also configured so that the combined compressed and purified stream is cooled by indirect heat exchange with the nitrogen product stream, the waste stream after having subcooled the first oxygen-enriched stream and the second part of the vapor phase stream. 
     
     
         17 . The apparatus of  claim 16 , wherein:
 the main heat exchanger is configured such that the waste stream and the nitrogen product stream indirectly exchange heat with the compressed and purified stream, the waste stream partially warms within the main heat exchanger and an exhaust stream fully warms within the main heat exchanger to refrigerate the apparatus; and   an expander is connected to the main heat exchanger so that the waste stream after having partially warmed is expanded within the expander with the performance of work to generate the exhaust stream.